Telecommunication cables are often comprised of a core that is overlayed with a metallic shield and an outer plastic jacket. The core itself is formed of a number of individually twisted insulated wire pairs or quads tightly bundled together and covered with a core wrap. The metallic shield is typically corrugated to provide cable flexibility and to protect the core from both mechanical and electrical damage. The plastic jacket, which is normally formed by extruding polyethylene onto the metallic shield, provides a moisture penetration barrier as well as additional mechanical protection for the core.
Recent cable configurations have employed a sealed, longitudinally split tubular shield having its longitudinal side portions overlapped to form a seam with adhesive bonding the two overlapped portions together. One such cable configuration is comprised of corrugated aluminum coated on each side with an ethylene acrylic acid copolymer. The overlapped seam portions of this type of shield are adhesively bonded together by the use of a hot melt or molten ethylene acrylic acid type adhesive.
One system for forming a sealed shield of the type just mentioned is disclosed in U.S. Pat. No. 4,035,211 which is assigned to the assignee of the present invention. Here, a cable core is passed through a belt former and overlap die partially overlayed with a strip or tape of corrugated aluminum. Within the belt former and die the copolymer coated aluminum shielding material is formed so as to encircle the cable core completely and to bring its opposite, longitudinal side portions into a configuration forming an overlapped seam. After exiting the belt former and die the shielded core is routed through a seam separator which reopens the seam. This is followed by passing the core and shield aside an extruder which applies adhesive onto one side portion of the now separated seam. Following this the shielded core is routed through two rollers which press the overlapped seam portions back together. Later, a polyethylene jacket is extruded onto the sealed shield.
Today some telecommunication cables, termed "Stalpeth cables", are being constructed for underground duct deployment which need to be shielded with metal offering higher strength than does aluminum or copper. Thus, their outer shields are formed of corrugated steel. The steel overlays rather than replaces the aluminum shield in order to retain electrical protection for the core. Again, the steel is preferably chemically bonded to a polyethylene jacket by the use of a layer of ethylene acrylic acid copolymer to provide buckling and moisture diffusion resistance. However, since steel is much less ductile than aluminum or copper, it possesses a very substantial degree of spring back. Thus, where it has been formed into a tubular configuration about a cable core with an overlapped seam, it has been difficult to reopen and to spread the seam apart in order to apply the adhesive. Accordingly, there exists a need today for a method and apparatus for providing a sealed shield about a cable core that overcomes the just described problems. It is to this task to which the present invention is primarily directed.